The present invention relates to a field of stimulable phosphor panel technology. In particular, the present invention relates to a high quality stimulable phosphor panel having a stimulable phosphor layer without deterioration of characteristics due to moisture absorption, and to a method of producing the stimulable phosphor panel.
There are known a class of phosphors which accumulate a portion of applied radiations (e.g. x-rays, α-rays, β-rays, γ-rays, electron beams, and uv (ultraviolet) radiation) and which, upon stimulation by exciting light such as visible light, give off a burst of light emission in proportion to the accumulated energy. Such phosphors called stimulable phosphors are employed in medical and various other applications.
An exemplary application is a radiation image information recording and reproducing system which employs a stimulable phosphor panel having a film formed of the stimulable phosphor (stimulable phosphor layer). The film is hereinafter referred to simply as a phosphor layer and the panel is hereinafter referred to simply as a phosphor panel or sometimes as a radiation image converting sheet. This radiation image information recording and reproducing system has already been commercialized as FCR (Fuji Computed Radiography) from Fuji Photo Film Co., Ltd.
In that system, a subject such as a human body is irradiated with x-rays or the like to record radiation image information about the subject on the phosphor panel (more specifically, the phosphor layer). After the radiation image information is thus recorded, the phosphor panel is scanned two-dimensionally with exciting light such as laser light to produce stimulated emission which, in turn, is read photoelectrically to yield an image signal. Then, an image reproduced on the basis of the read image signal is output as the radiation image of the subject, typically to a display device such as CRT or on a recording material such as a photographic material.
The phosphor panel is typically produced by the steps of first preparing a coating solution having the particles of a stimulable phosphor dispersed in a solvent containing a binder, etc., applying the coating solution to a support in panel form that is made of glass or resin, and drying the applied coating.
Phosphor panels are also known that are made by forming a phosphor layer on a support through methods of gas phase deposition (vapor-phase film formation) such as vacuum evaporation or sputtering, as disclosed in JP 2789194 B and JP 5-249299 A. The phosphor layer prepared by the gas phase deposition has excellent characteristics. First, it contains less impurities since it is formed under vacuum; in addition, it is substantially free of any substances other than the stimulable phosphor, as exemplified by the binder, so it has high uniformity in performance and still assures very high luminous efficiency.
One factor for deterioration of characteristics of the phosphor panel is moisture absorption by the stimulable phosphor layer.
The stimulable phosphor layer, in particular, an alkali halide-based stimulable phosphor layer having favorable characteristics, has high moisture absorption property and easily absorbs moisture even in a normal environment (normal temperature/normal humidity). As a result, deterioration of sharpness of a reproduced image or the like occurs due to deterioration of photostimulated luminescence characteristics, that is, sensitivity, or deterioration of crystallinity of the stimulable phosphor (destruction of columnar crystals in a case of an alkali halide-based stimulable phosphor having a columnar structure, for example).
In order to solve such problems, the phosphor layer is sealed with a moisture-proof member in the phosphor panel.
For example, JP 2677822 B discloses a phosphor panel having a phosphor layer sealed with a moisture-proof protective layer in which deterioration of characteristics of the phosphor panel due to moisture is prevented by: providing a sealing member on the periphery of the phosphor layer; and filling a dry gas into a space formed by a support, the protective layer, and the sealing member.
Further, JP 2886165 B discloses a phosphor panel having a stimulable phosphor layer similarly sealed with a protective layer, including: a protective layer holding member provided to surround a peripheral portion of the stimulable phosphor layer; and a low refractive index layer, which has a lower refractive index than that of the protective layer, provided between the stimulable phosphor layer and the protective layer. The stimulable phosphor layer is formed in a space blocked from the external atmosphere by the protective layer, the protective layer holding member, and a support.
The phosphor panels disclosed in JP 2677822 B and JP 2886165 B can prevent deterioration of characteristics due to moisture absorption by the phosphor layer to some extent.
However, the inventors of the present invention have conducted studies and have found that sufficient moisture-proof property cannot be attained only by sealing of the phosphor layer with a moisture-proof member, particularly under severe conditions such as high humidity and high temperature. The moisture-proof property is not yet sufficient for obtaining a phosphor panel having excellent characteristics such as providing a reproduced image with high sensitivity and favorable sharpness over a long period of time.